Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department

Superdiamagnetism,

In a weak applied field, a superconductor "expels" nearly all
magnetic flux. It does this by setting up electric currents near its
surface. The magnetic field of these surface currents cancels the
applied magnetic field within the bulk of the superconductor. As the
field expulsion, or cancellation, does not change with time, the
currents producing this effect (called persistent currents) do not decay
with time. Therefore the conductivity can be thought of as infinite: a
superconductor.

Near the surface, within a distance called the London penetration depth, the magnetic field is not completely cancelled. Each superconducting material has its own characteristic penetration depth.

Any perfect conductor will prevent any change to magnetic flux passing through its surface due to ordinary electromagnetic induction at zero resistance. The Meissner effect is distinct from this: when an
ordinary conductor is cooled so that it makes the transition to a
superconducting state in the presence of a constant applied magnetic
field, the magnetic flux is expelled during the transition. This effect
cannot be explained by infinite conductivity alone. Its explanation is
more complex and was first given in the London equations by the brothers Fritz and Heinz London.
It should thus be noted that the placement and subsequent levitation of
a magnet above an already superconducting material does not demonstrate
the Meissner effect, while an initially stationary magnet later being
repelled by a superconductor as it is cooled through its critical
temperature does.